Shock wave lithotripsy (SWL) has traditionally been used to treat kidney stone disease. There exist various devices and methods for generating high-intensity, focused shock waves for the fragmentation of concretions, such as kidney stones, inside a human being and confined in a body liquid. U.S. Pat. No. 3,942,531 of Hoff et al. discloses the use of a spark gap discharge in water to generate a shock wave within an ellipsoidal reflector which couples and focuses the shock wave to fragment kidney stones inside the body. Hahn et al. in U.S. Pat. No. 4,655,220, disclose a device using a coil and a mating radiator, in the form of a spherical segment, to produce magnetically induced self-converging shock waves. Wurster et al. in U.S. Pat. Nos. 4,821,730 and 4,888,746, disclose the use of piezoelectric elements arranged in mosaic form on a spheroidal cap to produce focused high-intensity shock waves at the geometric center of the cap, where the concretion must be placed. Other techniques for generation of shock waves for use in medicine include optical sources such as lasers, high-velocity projectiles, and localized explosive devices.
Inspired by the success of shock wave lithotripsy in the treatment of kidney stone disease, significant efforts have been made to explore a broad spectrum of shock wave applications in medicine. Lithotripter-generated shock waves have been investigated for potential use in tumor therapy, fracture healing of bones, treatment of tendinitis, and ablation of liver tissues with various degree of success.
It has also been shown that at low dosage ("low dosage" is defined for the purpose of this application as a low number of shock waves), lithotripter shock waves or pulses can cause a transient increase in cell membrane permeability without killing the cells. (It is recognized that the term "shock wave" is sometimes used herein interchangeably with the term "pulse.") In this regard, the invention recognizes that shock waves may facilitate the transfer of macromolecules into target cells and thus could potentially provide a non-invasive physical method for drug delivery and gene transfer. The invention also recognizes that enhanced cytotoxicity of several anticancer drugs in vivo by SWL may indeed benefit from shock wave-induced transient membrane permeability.
A typical or standard lithotripter pressure waveform (also referred to herein as a standard lithotripter shock wave) at the lithotripter focus consists of a leading shock front (compressive wave) with a peak positive pressure up to 100 mega Pascal's (MPa), followed by a tensile (negative) phase with a peak negative peak pressure up to 10 MPa, and a total pulse duration of 3 to 7 .mu.s. It is also known that the negative phase of an incident shock wave can induce transient cavitation bubbles in the focal region, if the tensile stress exceeds about 1 MPa.
Using the Gilmore model for bubble dynamics, Church has shown that a cavitation nucleus (1.about.10 .mu.m in radius) in water impinged by a lithotripter shock wave will be initially compressed by the leading shock front, and then expanded by the ensuing tensile wave into a bubble of 1.about.3 mm in diameter in a few hundred microseconds. Subsequently, the expanded bubble will undergo a violent inertial collapse, generating high temperature (up to 10,000 K) inside the collapsed bubble and secondary shock wave emission into the surrounding fluid. Following this primary collapse, the bubble will oscillate (rebound and then collapse again) several times with exponentially decreased amplitude before it eventually reaches a size of about 40 .mu.m due to rectified gas diffusion. The basic features of such a characteristic bubble oscillation has been confirmed experimentally, using simultaneous high-speed photography and acoustic emission measurements. During SWL, if bubbles generated by the earlier shock waves were to be stabilized on a tissue surface, the interaction of such a stable bubble with a subsequent lithotripter pulse may generate a liquid jet along the wave propagation direction, provided that the size of the bubble is within a certain range (250 .mu.m&lt;R.sub.b0 &lt;750 .mu.m, for a XL-1 lithotripter) as discussed in: A. Philipp, M. Delius, C. Scheffczyk, A. Vogel, and W. Lauterbom, Interaction Of Lithotripter Generated Shock Waves In Bubbles, J. Acoust. Soc. Am. 93;2496-2509 (1993), which is deemed incorporated herein by reference.
Also recognized by the invention is the belief that the shear stresses, and secondary shock wave emission and jet impact associated with the rapid expansion and collapse of cavitation bubbles may contribute to the bioeffects produced by SWL. When cavitation activity in the culture medium is suppressed by excessive ambient pressure, SWL-induced cell injury and membrane permeability change can be significantly inhibited. In contrast, when cavitation activity in vivo is enhanced by intravenous injection of ultrasound contrast agents (well-known cavitation nuclei) immediately before SWL, the vascular injury produced in animal models is substantially increased, even at low-pressure amplitudes ineffective for stone fragmentation. These findings are recognized by the invention to clearly demonstrate that cavitation is an important mechanism for SWL-induced bioeffects.
Despite all this evidence, the pressure waveform and associated cavitational activities produced by current shock wave lithotripters may not be optimal for tumor treatment and macromolecule delivery. Several studies have shown that an air-water interface near the lithotripter focus can dramatically enhance SWL-induced bioeffects on cells and small tumors. This observation had led some investigators to suggest that a different shock waveform is needed for tumor therapy. Moreover, the transfection efficiency of shock wave-mediated gene transfer is currently low compared to other established methods, and air injection into the target cells was found to be necessary to enhance the transfection efficiency in vivo. Flotte et al, in U.S. Pat. No. 5,614,502, disclose use of compounds in combination with a series of high-pressure transients for delivery of such compounds into cells. However, Flotte et al use a high-pressure shock wave without controlling the resulting cavitation, resulting in high cell death. With this in mind, the present invention recognizes that the ability to control the formation and subsequent bubble oscillations is critical for producing optimal bioeffects by SWL. However, because of the temporal profile and the low pulse repetition rate of current lithotripter shock waves, the invention also recognizes that the collapse of SWL-induced cavitation bubbles is uncontrolled and is predominantly influenced by the inertial effect of the surrounding fluid. Furthermore, what is also recognized is that because of the limited fluid-filled space in tissue and in blood vessels, the expansion of SWL-induced cavitation bubbles in vivo can be severely constrained, and thus the resultant bioeffects are less dramatic as compared to in vitro conditions.
With the foregoing in mind, it becomes a general object of the present invention to provide an apparatus and method for delivery of macromolecules into living cells.
It is a further object of the present invention to provide an apparatus and method for applying to living cells a first low-pressure shock wave followed a few microseconds later by a high-pressure shock wave in order to facilitate delivery of macromolecules into the living cells.
It is another object of the present invention is to provide an ellipsoidal reflector for current electrohydraulic shock wave lithotripters that generates a first low-pressure shock wave followed a few microseconds later by a high-pressure lithotripter shock wave.
It is a further object of this invention to provide an ellipsoidal reflector that generates a first low-pressure shock wave followed a few microseconds later by a high-pressure lithotripter shock wave such that the peak pressure of the first shock wave may be varied.
It is yet another object of the present invention to use a preceding weak shock wave to induce inertial microbubbles in front of a high-pressure lithotripter shock wave.
It is still another object of the present invention to control the size and distribution of microbubbles in order to facilitate the delivery of macromolecules into living cells.
Another object of this invention is to provide an apparatus and method for manipulating shock wave and microbubble interaction to selectively improve the efficiency of shock wave-mediated macromolecule delivery and tissue ablation.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.